The present invention relates to the Internet and other packet-based networks and more particularly to methods for wireless access to packet-based networks by mobile devices.
Support for wireless access between a correspondent node and a mobile device over the Internet is outlined in an Internet Engineering Task Force (IETF) proposal entitled xe2x80x9cIP Mobility Support,xe2x80x9d C. E. Perkinsxe2x80x94Editor, Request for Comments 2002 (October, 1996; hereinafter xe2x80x9cMobile IPxe2x80x9d). By utilizing Mobile IP, each mobile device is identified by a fixed home address and associated home agent, regardless of its current point of attachment to the Internet. Packets sent to a mobile device, from a correspondent node, are directed to the home agent. If the mobile device is away from home, the home agent tunnels packets to a foreign agent associated with the mobile device.
An IP packets typically limited in size with 40 bytes of the packet utilized as the IP packet header. The IP header includes a source address and a destination address. The correspondent node is set as the IP header source address and the mobile device is set as the IP header destination address. The remainder of the limited packet size is available for data payload. When received at the node hosting the home agent, and when the mobile device is away from home, the home agent intercepts the IP packet and encapsulates the IP packet with appended IP header destination and source addresses, and forwards the encapsulated packet in an IP-in-IP tunnel to the foreign agent. The encapsulated packet is comprised of the original 40 byte IP header, a ten byte appended IP header source address designated with the home agent""s IP address, and a ten byte appended IP header destination address designated with the foreign agent""s IP address, with the remainder of the limited packet size available for data payload. The data payload portion of an encapsulated packet is therefore 20 bytes less than that available for data payload in a packet which is not encapsulated for tunneling. The encapsulated packet is then tunneled to the foreign agent. The foreign agent strips the appended IP header source and destination addresses and delivers the remainder of the packet to the mobile device for processing.
Inclusion of the encapsulating addresses used for tunneling causes at least two undesirable effects. The first effect, as noted above, is a reduction in the available data payload portion size since the addition of the encapsulating header addresses decreases packet transmission efficiency due to additional included packet overhead. Second, when a packet of maximum size is intercepted at the home agent for tunneling to the mobile device, the home agent returns an Internet Control Message Protocol (ICMP) error message back to the correspondent node to indicate that the addition of a tunneling header would require fragmentation. The correspondent node then limits the data payload size of the packet and retransmits. Therefore, an additional round trip for packets conveyed from the correspondent node to the home agent is required. This effect may be especially noticeable when utilizing the Mobile IP tunneling scheme for a web transfer from a correspondent node to a mobile device, resulting in an additional delay of 500 milliseconds or more, since each web page transfer may require a plurality of Transmission Control Protocol (TCP) downloads to complete the transfer.
Packet delay and tunneling overhead are reduced when packets are forwarded from a home agent to a mobile device""s foreign agent by substituting the packets"" destination addresses at the home agent, rather than encapsulating each packet with an additional source and destination address. In an embodiment of the invention, the foreign agent is co-located with the mobile device. When a mobile device acquires a new foreign agent, the mobile device notifies the home agent as to the address corresponding to the foreign agent. Packets received at the home agent having the mobile device as a packet header destination address are not encapsulated with additional source and destination addresses. Rather, the packet header is parsed, the foreign agent address is substituted for the mobile device address, and the packet is forwarded to the foreign agent. The foreign agent, upon receiving the packet, removes the foreign agent address and replaces the mobile device address as the packet header destination address. The packet is then forwarded to the mobile device.
Interchanging the packet header destination addresses as described provides two beneficial effects. First, since encapsulating header addresses are not required nor used, the packet header size for packets forwarded from the home agent to the foreign agent is not increased. Therefore, packet transmission efficiency is improved when compared to the scheme utilized for Mobile IP tunneling. Second, since the packet payload size is not reduced for packets forwarded from the home agent to the foreign agent, the home agent does not return an ICMP error message to the correspondent node and therefore does not require retransmission of a packet having a reduced data payload size.